1. Field
One embodiment of the present invention relates to an immersion objective optical system, exposure apparatus, device fabrication method, and boundary optical element and, more particularly, to a projection optical system applicable to exposure apparatus used in fabrication of devices such as semiconductor devices and liquid-crystal display devices, by photolithography.
2. Description of the Related Art
The photolithography step for fabrication of semiconductor devices or the like is carried out using an exposure apparatus for projecting a pattern image of a mask (or reticle) onto a photosensitive substrate (wafer, glass plate, or the like coated with a photoresist) through a projection optical system to effect exposure thereof. In the exposure apparatus, the demanded resolving power (resolution) of the projection optical system is becoming higher and higher with improvement in the degree of integration of the semiconductor devices or the like.
For improving the resolving power of the projection optical system, it is possible to decrease the wavelength λ of illumination light (exposure light) and to increase the image-side numerical aperture NA of the projection optical system. Specifically, the resolution of the projection optical system is expressed by k·λ/NA (where k is a process factor). Furthermore, the image-side numerical aperture NA is expressed by n·sin θ, where n is the refractive index of the medium (normally, gas such as air) between the projection optical system and the photosensitive substrate and θ a maximum angle of incidence to the photosensitive substrate.